ROM Cartridge Address Decoding
4 messages · 2015-08-28 → 2015-08-28 · Yahoo Group era · View archive on archive.org
Participants: Mark J. Blair, Al Hartman
Preserved from the Timex/Sinclair 2068 Yahoo Group (2001–2019), which is no longer online. Text reproduced from the archive.org archive; email addresses masked.
Messages
1. ROM Cartridge Address Decoding
Mark J. Blair · Thu, 27 Aug 2015 22:53
I wonder if anybody here might know more about address decoding on the TS2068 cartridges. In the pictures I've seen of naked OS-64 cartridges, I see that there's a demultiplexer chip, presumably to decode the upper 2-3 address bits in order to place the ROM at the desired boundary in the dock address space. It looks like the card has provisions to change the bank, too. I haven't dismantled Rod's OS-64 cartridge to fully trace out the wiring, but that's my best guess based on pictures.
As I understand it, the TS2068 might look at different locations for a recognizable signature for one style or cartridge or another. At least some types are expected to live at 0x0000, right?
Now, Paul says that his EPROM + joystick card doesn't include that decoding, so I infer that when he installed a 27128 with OS-64 on it, it appeared mirrored at each 16k boundary. And it seemed to work, to the extent that it booted up with text near the bottom of the screen that's too small to read on a TV, just like the original OS-64 cartridge did for me (and that's as far as I tested it).
So here's my question: Does a minimal EPROM cartridge really have any need for the upper address decoding? Or is it adequate to let it appear at 0x0000, and then mirrored through the entire 64k address range? If I understand the tech manual properly, it seems to me that the cartridge is all by itself in the separately decoded "dock" address space, so it shouldn't conflict with anything if there's just the one cartridge in the slot of a regular TS2068.
I saw the sample EPROM card design in the technical manual which includes a demultiplexer to map four 27128 EPROMs into the 64k dock address space. But for a simple single-EPROM cartridge, can one safely get away with ignoring the upper bits and letting it mirror all over the place?
--
Mark J. Blair, NF6X <[email]>
http://www.nf6x.net/
2. Re: [ts2068] ROM Cartridge Address Decoding
Al Hartman · Fri, 28 Aug 2015 02:21
That won't work in the case of a Spectrum Emulator Cartridge because if you
put a Twister on the computer and try to use a Zebra Disk System, the Black
Spectrum Interface has a ROM in it, so it won't work using Paul's cartridge.
And that also means you can't use OS-64 as it was intended to be used as a
companion to the Zebra FDD-3000 in TS-2068 mode as it will step all over the
expansion ROM in the silver interface box.
If you usePaul's board with a twister with an Interface One from Sinclair to
attach the Wafer tape drive, it has a ROM in it that also expects to map in
which won't work either.
If you have a Stringy floppy drive, you won't be able to use it with OS-64,
because the OS-64 will then walk all over the stringy floppy ROM area.
In fact, no third party disk system like the Oliger or Aerco will work with
Paul's cartridge board inserted, I'd guess.
Paul's cartridge is good to use just for what it is. As a Spectrum Emulator
in a tape-based system. But, I don't think you could add-on anything else to
it that has firmware. Like the DivMMC using a Twister.
Just my opinion upon reflection. Stewart fully decoded the address space
because he always designed our products conservatively.
-[ Al ]-
-----Original Message-----
From: 'Mark J. Blair' [email] [ts2068]
I wonder if anybody here might know more about address decoding on the
TS2068 cartridges. In the pictures I've seen of naked OS-64 cartridges, I
see that there's a demultiplexer chip, presumably to decode the upper 2-3
address bits in order to place the ROM at the desired boundary in the dock
address space. It looks like the card has provisions to change the bank,
too. I haven't dismantled Rod's OS-64 cartridge to fully trace out the
wiring, but that's my best guess based on pictures.
As I understand it, the TS2068 might look at different locations for a
recognizable signature for one style or cartridge or another. At least some
types are expected to live at 0x0000, right?
Now, Paul says that his EPROM + joystick card doesn't include that decoding,
so I infer that when he installed a 27128 with OS-64 on it, it appeared
mirrored at each 16k boundary. And it seemed to work, to the extent that it
booted up with text near the bottom of the screen that's too small to read
on a TV, just like the original OS-64 cartridge did for me (and that's as
far as I tested it).
So here's my question: Does a minimal EPROM cartridge really have any need
for the upper address decoding? Or is it adequate to let it appear at
0x0000, and then mirrored through the entire 64k address range? If I
understand the tech manual properly, it seems to me that the cartridge is
all by itself in the separately decoded "dock" address space, so it
shouldn't conflict with anything if there's just the one cartridge in the
slot of a regular TS2068.
I saw the sample EPROM card design in the technical manual which includes a
demultiplexer to map four 27128 EPROMs into the 64k dock address space. But
for a simple single-EPROM cartridge, can one safely get away with ignoring
the upper bits and letting it mirror all over the place?
--
Mark J. Blair, NF6X <[email]>
http://www.nf6x.net/
------------------------------------
Posted by: "Mark J. Blair" <[email]>
------------------------------------
------------------------------------
Yahoo Groups Links
3. Re: [ts2068] ROM Cartridge Address Decoding
Mark J. Blair · Thu, 27 Aug 2015 23:39
Ah, I see. Well, then maybe there's still a use for a different ROM cart that's not fulfilled by Paul's neat ROM + Kempston card, after all?
4. Re: [ts2068] ROM Cartridge Address Decoding
Al Hartman · Fri, 28 Aug 2015 03:07
Paul never responded with a link to what he referred to as Spectrum MOD B
code. But, I found this file.
We never experienced a problem using real Spectrum ROMs because the 2068
would start using the 2068 ROM which would initialize the hardware, then we
would issue the out command that would swap the ROM and reset the machine
into Spectrum mode. That worked fine for us.
If you wanted to make a card as you describe that would be great. I'd be
glad to pay you a per board royalty for each board I sell. You can put it on
Oshpark as a public project like the CocoROMpak board.
-[ Al ]-
-----Original Message-----
From: 'Mark J. Blair' [email] [ts2068]
Ah, I see. Well, then maybe there's still a use for a different ROM cart
that's not fulfilled by Paul's neat ROM + Kempston card, after all?
Indexed under
Cartridges, EPROM & dumping · Hardware projects & new boards